This one is simple — I threw Space: 1889 and Castle Falkenstein together, borrowing from each for the setting, but mostly it was mating the setting of the former to the rules of the latter. I might be doing it again with Space: 1889 and Revelations of Mars for the Hollow Earth Expedition game.
To do this, I think I’m going to set it in the 1930s, but use some of the Martian races from both books, make Mars more the world of 1889, but with the great machine idea of RoM as the central conceit. It gives me rockets, Nazis, commies, and all the stuff I like from Space: 1889, but with the more Barsoomian feel of Revelations. Now the question is what rules set? Ubiquity is the obvious choice, but I think Atomic Robo’s stripped down Fate will work, as well.